Lane said during an interview on MSNBC's "Morning Joe" that half of the billionaires on the Forbes 400 list lie about their net worth. Trump is notorious for quibbling with writers who he has said underestimate the value of his business empire and brand, which he says totals $10 billion.

But Lane said the Forbes team spent more time crunching Trump's numbers this year than it had for any other billionaire in the magazine's history, going over each asset with the real estate mogul himself to come up with a definitive figure for the Forbes 400 list: $4.5 billion.

The discrepancy between Trump's estimate of his own net worth and the numbers that news organizations have placed on his empire lies with the value of Trump's personal brand, Lane said.

"He puts the biggest value on his brand," the Forbes editor explained. "He says 'My brand is worth $3-$5 billion dollars,' just the Trump brand. We think that the money he makes is based on his brand. We think that's factored into the deals he already cuts so as a policy we do not count that."

Lane added that Forbes talked to 20 brand experts who estimated the worth of the Trump brand anywhere from $125 million to $1.1 billion. Those experts told the magazine that Trump's summer surge in the Republican presidential primary polls actually grew the value of billionaire's brand despite the fallout from his disparaging comments about immigrants crossing the U.S. border with Mexico.